Field of the Invention
The present invention relates to a subject information obtaining apparatus, a method for obtaining subject information, and a program, and more particularly to a technique for obtaining subject information by transmitting an elastic wave to a subject and receiving a wave reflected inside the subject.
Description of the Related Art
In general, in an ultrasonic diagnostic apparatus as a subject information obtaining apparatus, the spatial resolution in the depth direction when image data is formed by a pulse-echo method can be represented by an expression (nλ)/2, where λ denotes the wavelength of an ultrasonic wave, which is an elastic wave, and n denotes the number of waves transmitted. For example, when two wavelengths of an ultrasonic wave having a center frequency of 12 MHz is transmitted, the spatial resolution in the depth direction is about 0.13 mm.
The pulse-echo method will be described. First, when an ultrasonic pulse has been transmitted to a subject, an ultrasonic wave is reflected and comes back in accordance with differences in acoustic impedance inside the subject. Next, the reflected wave is received and image data is generated using a received signal of the reflected wave. Typically, an envelope of the waveform of the received signal is obtained and converted into values of luminance, in order to generate the image data. By displaying an obtained image, luminance information on a scan line in a direction in which the ultrasonic wave is transmitted and received can be obtained. By obtaining the luminance information on each scan line for a plurality of times, that is, by repeating transmission and reception of an ultrasonic wave in a plurality of directions or positions in the subject, the inside of the subject can be imaged.
Although it is possible to realize a value of the spatial resolution in the depth direction of about 0.13 mm by using the pulse-echo method, higher values of spatial resolution are required. For example, if the layer structure of the blood vessel walls of a carotid artery can be observed in more detail, it is possible to contribute to early detection of arteriosclerosis or the like.
As techniques for improving the spatial resolution in the depth direction, a frequency-domain interferometry (FDI) method and a Capon method, which is a type of adaptive signal processing, are used in “Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina, and Toru Sato: Conf Proc IEEE Eng Med Biol Soc. 2010; 1: 5298-5301”, in order to present results of imaging of the layer structure of blood vessel walls. By using the FDI method and the Capon method for received signals, it is possible to further improve the spatial resolution in the depth direction (scan line direction). However, a plurality of reflection layers are supposed to exist in a range (processing range) of a signal in the depth direction that has been cut out in order to execute the processing of the FDI method. In addition, it is probable that a plurality of waves reflected from reflection layers that are located close to one another have a high correlation. It is known that if the adaptive signal processing such as the Capon method is directly adopted for received signals of a plurality of such reflected waves that have a high correlation, unexpected effects such as cancellation of a desired signal can be produced. The effects caused by waves (coherent interference waves) that have a correlation can be reduced (suppressed) by using a frequency-averaging technique, and the FDI method and the Capon method can be adopted for the received signals of reflected waves.
However, if the FDI method and the adaptive signal processing are adopted for the received signals of reflected waves, how strongly coherent interference waves are suppressed does not match between adjacent scan lines, since the processing is executed for each scan line. It has been found that, as a result, it is possible that there are portions of obtained image data in which the continuity in a direction that intersects with the scan lines is insufficient.
On the other hand, the spatial resolution in the direction that intersects with the scan lines varies depending on convergence conditions at the times of transmission and reception of an elastic wave. In a general pulse-echo method, in order to complete imaging without missing minute reflection bodies in an observation region in a subject, distances between the scan lines (the intervals of the scan lines) are set to be shorter than the spatial resolution in the direction that intersects with the scan lines. Therefore, it can be concluded that if the FDI method and the adaptive signal processing are not used, the continuity between adjacent scan lines does not become insufficient.
That is, by using the FDI method and the adaptive signal processing, the continuity in the direction that intersects with the scan lines can become lower than that of a general image (an image generated by obtaining envelopes of received signals). Accordingly, a unique problem is caused that when continuity becomes lower, visibility also becomes lower.